The in situ behaviour of n-hexane and 1-hexene over ultrastable Y zeolite was studied quantitatively using thermogravimetric equipment at low (323–423 K), middle (523–623 K), and high (623–773 K) temperatures. At low temperatures, adsorption is the absolute dominant cause of catalyst weight change when either n-hexane or 1-hexene come in contact with US-Y, whose amount decreases with temperature. At middle temperatures, adsorption is also the dominant cause of catalyst weight change in the case of n-hexane, with the adsorbed amount, however, about one order of magnitude lower than at low temperatures, while for 1-hexene, oligomerisation from the olefinic reactant becomes the dominant cause of catalyst weight change. At high temperatures, oligomerisation from the olefinic products of catalytic cracking becomes the dominant cause of catalyst weight change for both cases, n-hexane as well as 1-hexene, while the extent of coke formation increases with temperature.